1. Field of the Invention
The invention relates to a method for heated tool butt welding of pipe and tube of thermoplastic materials which are used above all for waste water, fresh water, milk, beer, etc., for the transportation of solids and for the protection or for cooling electrical cables.
2. Discussion of the Prior Art
The heated tool butt welding is the dominant method of joining polyethylene and polypropylene pipe which is chiefly used for the purposes mentioned above (Plastverarbeiter, 1977, page 642). Welding of pipe of thermoplastic materials is effected by methods which are prescribed in instructions (e.g., in specification Deutscher Verband fur Schweisstechnik No. 22 07 for rigid polyethylene, welding instructions for laymen).
For heated tool butt welding, the joint surfaces of the parts to be welded are placed into contact without or with pressure, thereafter heated to the welding temperature with reduced pressure and fitted together after removal of the heated tool.
The heating element or tool usually consists of an electrically heated metal disk. After having been heated to the welding temperature, the heated tool is placed between the parts to be welded, and the surfaces to be joined are pressed against the heated tool on both sides. The contact pressure is maintained until the surfaces to be joined are in complete engagement with the heated tool. At this time, a bead is present throughout the circumference of the parts to be welded. During the heating time which now follows, the contact pressure is reduced to a pressure in the range of 0.2 to 0.5 bars. After heating which requires about 30 to 250 seconds depending upon the wall thickness of the pipe, the surfaces to be joined are removed from the heated tool. When removing the heated tool, care is taken that the surfaces to be joined are not damaged or contaminated. The surfaces to be joined are fitted together immediately after having removed the heated tool.
The pressure set for making the joint is increased according to Specification DVS No. 22 07 and maintained. It is maintained until the weld has cooled. Unclamping can be effected only after cooling. The bead which is formed in welding has the appearance shown in FIG. 1 if the weld joint was made satisfactorily.
The inner bead which is necessarily formed with this mode of operation is regarded as disturbing. It increases the pressure loss, promotes deposits of solids, hinders the drawing-in of cables into protective pipe, represents an abrasive point for cables having been drawn in, etc.
Subsequent mechanical removal of the inner bead involves a considerable expense and is hardly possible in case of pipe lines of greater length due to their out-of-round.
It is an object of this invention, therefore, to provide a process for welding together by butt welding ends of thermoplastic pipe wherein, during the butt welding operation, the inner bead that is formed is flattened in such a manner that, on the one hand, the disadvantages mentioned above and encountered in the case of a pronounced inner bead are avoided but that, on the other hand, the strength properties of the weld joint are not impaired.